1. Field of the Invention
The present invention relates to a Vertical Cavity Surface Emitting Laser (VCSEL) that emits laser light from the top face, particularly to a VCSEL suitably applicable to purposes necessitating transverse mode control.
2. Description of the Related Art
A VCSEL emits light in the direction orthogonal to a substrate differently from the existing edge-emitting laser diodes. In the VCSEL, many devices are able to be arranged in a state of a two dimensional array on the same substrate. Therefore, the VCSEL has recently attracted attention as a light source for a digital copy machine or a printer.
In the past, in the foregoing type of VCSEL, a pair of multilayer film reflectors is formed over a semiconductor substrate, and an active layer as a light emitting region is provided between the pair of multilayer film reflectors. In one of the multilayer film reflectors, in order to improve efficiency of current injection into the active layer and lower the threshold value current, a current confinement layer having a structure in which a current injection region is confined is provided. Further, a lower electrode is provided on the bottom face side, and a p-side electrode is provided on the top face side. The p-side electrode is provided with a light emitting aperture to emit laser light. In the VCSEL, a current is confined by the current confinement layer, and then injected into the active layer where light is emitted. While the emitted light repeats reflection between the pair of multilayer film reflectors, the light is emitted as laser light from the light emitting aperture of the p-side electrode.
In general, in the foregoing VCSEL, high-order transverse mode oscillation is easily generated. Therefore, for example, in Japanese Unexamined Patent Application Publication No. 2004-119582, a reflectance adjustment layer is provided in the central part of a light emitting aperture of the top face of a mesa, a region where fundamental transverse mode is mainly shown is set to a region with high reflectance, and a region where high-order transverse mode is mainly shown is set to a region with low reflectance. Thereby, oscillation of the high-order transverse mode is prevented, and single transverse mode oscillation is realized.